Mixing between a reference signal and a data signal is often necessary to extract information about an optical device. A probe signal and a reference signal originating from the same source are typically mixed, resulting in fringes that can be detected and used to assess information about the device being probed. In interferometric sensing, a reference signal is mixed with a signal whose phase and/or amplitude is modified by a parameter to be measured.
The mixing produces an interference signal, and the amplitude of the interference signal depends on how efficiently the two optical signals mix. When the two signals have the same polarization state, the mixing efficiency is 100%. When the two signals have orthogonal polarization states, no mixing occurs—0% efficiency. Between these two limits, only the portion of the signals whose polarization states resolve onto a single polarization axis actually mix. The reduced, mixed-signal amplitude results from an unmixed component in an orthogonal polarization state. This inefficiency is usually referred to as polarization induced fringe fading.
Polarization diversity detection overcomes polarization induced fading. One commonly known interferometric scheme that can suffer from polarization fading is Optical Frequency Domain Reflectometry (OFDR). OFDR injects a highly monochromatic beam of light into the optical system or device to be tested. The frequency of that light is varied slowly with a time-linear sweep, and the optical signal back-scattered from the optical system is detected by coherently mixing the back-scattered signal with the reference input signal. The beat frequency component of the mixed signal, (corresponding to an interference signal), is measured to determine a position of the back-scattering (reflection) point in the optical system/fiber. The interference signal amplitude also determines a back-scattering factor and an attenuation factor for the reflected light.
U.S. Pat. Nos. 6,376,830 and 5,789,521 provide further details regarding OFDR measurement and are incorporated herein by reference. Reference may also be made to commonly-assigned, U.S. patent application Ser. No. 10/005,819, entitled “Apparatus and Method for the Complete Characterization of Optical Devices Including Loss, Birefringence, and Dispersion Effects,” filed on Dec. 14, 2001.
A single mode optical fiber supports two degenerate polarization modes. This degeneracy causes field energy to be transferred between the modes as they propagate down the fiber. This phenomenon causes the polarization fading in fiber-optic interferometers. FIG. 1 shows schematically a Mach-Zender interferometer. The arrows denote electric field (E) vector components. Polarization fading occurs whenever E1 and E2 are not co-linear, i.e., Ē1·Ē2=|Ē1∥Ē2|cos θ, θ≠0. The power measured at the detector is proportional to the square of the absolute value of (E1+E2). The interference terms of this relationship are proportional to E1·E2*+E2·E1*, where * denotes a complex conjugate. When a first coupler C1 splits the input field Ein, there is a chance that the split fields E1 and E2 in the respective interferometer arms evolve into orthogonal polarizations. As described above, in that situation, no interference fringes will be detected, and there is complete polarization fading or 0% mixing efficiency.
A worst case scenario in which the fields interfering on the detector, E1 and E2, are orthogonal is shown in FIG. 2. More formally, in some orthogonal basis, the fields can be written E1=(a, 0)exp(iωτ) and E2=(0, d), where τ is the propagation time difference between the two interferometer arms (τ=neL/c, where ne is the effective (modal) index of the fiber. The basis of a vector set includes two vectors in two dimensions or three vectors in three dimensions that are used to represent all other possible vectors. Knowing the basis of a vector set is essentially the same as knowing the coordinate system for a point in space. For example, a location may be described as being at 32 degrees North and 25 degrees West. The coordinate system is the set of latitude and longitude lines on the Earth, and the particular location is understood. The basis set is a pair of vectors, each one degree (60 nautical miles) long, with one vector pointed to the North and one vector pointed to the West.
Now in the S-P basis set, shown in FIG. 2 as orthogonal, the fields can be written as E1=(a′, b′)exp(iωτ) and E2=(c′, d′) so E1·E2=0, but E1+E2=(a′ exp (iωτ)+c′, b′ exp (iωτ)+d′). Polarization diversity detection detects the s and p components (or projections onto the s and p axes) of E1+E2 separately using two S and P detectors. The power at each detector is proportional to the modulus squared of the components of the total field:Ps∝|a′exp(iωτ)+c′|2  (1)Pp∝|b′exp(iωτ)+d′|2  (2)These diversity power signals exhibit fringes even though the total field, i.e., the sum of two orthogonal fields, does not.
Polarization diversity detection may be implemented using a polarizing beam splitter (PBS) as show in FIG. 3. If the field at the PBS is Ebs and is given by Ebs=(A, B) in the basis set of the polarizing beam splitter, then the measured powers at the S and P detectors are Ps∝|A|2 and Pp∝|B|2. When the PBS splits the field into different components, the crystalline structure of the PBS imposes an orthonormal basis onto which the incident field is projected. That orthonormal basis is needed to extract information contained in the E1 and E2 amplitudes.
But there are drawbacks with using polarizing beam splitters. First, they are bulky and expensive. Second, polarizing beam splitters add stray reflections to the detected signals. Third, if the polarizing beam splitter is designed to operate in a particular wavelength, e.g., 1500 nm, it cannot be easily and inexpensively altered to operate at a non-standard wavelength, such as 800 nm, at least as compared to a standard optical coupler. For these and other reasons, it is an object of the present invention to perform polarization diversity detection without a polarizing beam splitter.
The present invention performs polarization diversity detection without using a polarizing beam splitter. Field vectors from one interferometer arm are used as the basis upon which to project a field vector from the other interferometer arm. Polarization diversity detection is performed using only standard optical couplers, e.g., 50-50 couplers. A polarization beam splitter is not needed.
A first coupler receives a first optical signal from a device or system under test and generates first and second coupler outputs. A second coupler receives a second optical signal from a reference source and generates third and fourth coupler outputs. A first polarization controller (PC) changes the polarization state of the third coupler output and generates a PC output. A third coupler generates a first combined output from the first coupler output and the PC output. A fourth coupler generates a second combined output from the second coupler output and the fourth coupler output. A first detector detects a first power of the first combined output in a first projection plane, and a second detector detects a second power of the second combined output in a second projection plane. A processor processes interference terms in the first and second powers in the first and second projection planes to determine one or more characteristics of the first optical signal.
A second polarization controller changes the polarization of the first optical signal before it is received in the first optical coupler. The first and second polarization controllers are adjusted to calibrate the fiber optic measurement device. Different second polarization controller settings result in multiple corresponding vector measurements at the first and second detectors. The processor calculates a vector calibration matrix using these vector measurements. The processor corrects subsequent detected vector measurements using the vector calibration matrix. The corrected vector measurements ensure that the vector representation of the first optical signal are in an ortho-normal basis set.
The OFDR components can be constructed simply using optical fiber, and if desired, from the same type of standard low-loss fiber. Matching the type of fiber throughout the optical network results in very low losses with essentially zero scattering events in the network. As a result, the OFDR produces very clean time domain measurements (only reflection events from the device under test appear).
Another advantage of fiber-based OFDR construction is significant cost reduction and increased reliability and flexibility. A polarization controller can be implemented simply as a single loop of fiber that is moved to achieve a certain polarization state at the output. Once the loop is positioned, it need not be moved again. Couplers are constructed by melting two optical fibers together. In order to manufacture couplers for operation at widely different wavelengths, (e.g., 615 nm and 1550 nm), coupler manufacturers need only purchase fiber (an inexpensive commodity) designed for that wavelength and melt two sections together using the same process for all wavelengths. No re-tooling or significant changes to the process are required. As a result, couplers are readily available at all wavelengths at a reasonable price in contrast to polarization beam splitters and other bulk-optic based optical components.
Other features, aspects, and advantages will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrated by way of example. Like reference symbols refer to like elements throughout.